Photonic beamforming techniques can be used for the optical control of phased arrays, for example in a space-based radar. A compact, directly modulated photonic transmitter that would exhibit large modulation bandwidths and low harmonic distortions is highly desirable in such an application.
Distributed feedback (DFB) lasers are key components for telecommunication networks and fiber based RF photonic systems. However, the bandwidth of commercial available DFB lasers is typically well below 10 GHz which will not be able to meet the needs of most near-future application. One way of improving the bandwidth to well beyond 10 GHz is to use external optical injection technique as theoretically predicted by J. Wang and G. Ybre and as experimentally demonstrated by X. J. Meng. Meng has reported the enhancement of bandwidth by a factor of 3.7 and reduction of second harmonic distortion by 10 dB. They also observed the narrowing of line-width in their experiments as well as the reduction of distortion products. See X. J. Meng, IEEE Trans. Microwave Theory & Technique, vol. 47, no. 7, pp 1172-1176 (1999).
However, the prior art technique requires the use of an external laser (e.g. a Ti-Sapphire laser) tuned to within 5-25 GHz of the slave DFB laser, to realize performance gains, which laser is bulky and adds expense. Furthermore, the use of two physically distinct lasers (as the master laser and slave laser) make their optimal conditions for injection locking highly vulnerable to environmental perturbations such as temperature variations.
The self-injection locking technique of the present invention offers the advantages of low cost and compactness combined with ruggedization in packaging. Most importantly, it offers thermal stability over the external injection technique reported by X. J. Meng et al. In particular, a self-injection locked DFB laser should achieve much improved performance stability with respect to environmental perturbations. According to the two-laser prior art technique, for an injection ratio of −15 dB, a differential frequency stability of ˜15 GHz between the master and slave laser must be maintained. This translates into a wavelength difference of ˜0.128 nm between the two lasers. To maintain this wavelength difference stable between two physically distinct lasers, well-engineered feedback loops must be utilized to control the physical parameters that determine their lasing wavelengths. In many instances, these parameters are the temperatures of the active junctions inside the master and slave lasers.
The self-injection approach of the present invention removes the disadvantage of using two distinct lasers in transmitter design. In particular, the disclosed approach taps part of the transmitter's optical output, and then takes advantage of external modulation to shift the frequency of the tapped signal. After filtering, this frequency-shifted signal is used to accomplish injection-locking of the DFB laser.